kirby3-canoe.bps + 109C = No change unfortunately.
Still freezes immediately when defeating the final boss stage 2.firemen1.ips + 0000 = Definitely on the right track!
The stage 4 "rain room" now properly displays, but with issues.
Other issues are now present as well.
I noticed something though.
When you enter a room, it begins as a black screen, then the middle opens up and expands outwards revealing the room.
It's basically like a "spotlight" effect that keeps expanding bigger until you can see the entire room.
For every room, this happens very quickly, except for the stage 4 rain room.
Without firemen1, it just stays at a black screen, but the game is still completely playable.
In fact, you can exit the rain room and everything will be normal, re-enter the rain room and black screen again, but still playable.
With firemen1, it now correctly expands to reveal the rain room.BUT
, this "expanding" takes a MUCH longer amount of time, and it flickers like CRAZY when it's expanding.
The rain room also occasionally flickers as a whole as well.
Very buggy room, especially compared to every other room in the game, but at the very least, it is correctly displaying now.
Another issue that this patch introduces is that the top horizontal line of the screen is cut off.
It is cut off MOST of the time, but it occasionally comes back for a very short while, only to get cut off again.
The other issue with this patch is that the character portrait during dialogue also has its top horizontal line cut off.
Here are some comparison pictures for the top line cutting of the screen and character portrait.
Canoe (firemen1): https://i.imgur.com/rBQjCYA.jpg
Higan (vanilla): https://i.imgur.com/JO8nYfi.jpg
Also, the Snes9x save state was from 1.55, I apologize for that. I should have used 1.51.
I went ahead and downloaded Geiger's Snes9x ep10r2 and created a save state immediately in front of the door to the rain room.
I used the firemen1 patched ROM. (Vanilla Firemen just dies in Geiger, gets stuck after the Human logo. But your old, first Firemen patch (for the sound issues) makes the game work in Geiger.)
Just in case if you still need it: